1. Field of the Invention
The invention relates to a transmission system for transmitting data signals in a modulation band. A transmitter and a receiver are coupled thereto by a transmission path. The transmitter comprises a modulator and a transversal smearing filter coupled thereto and the receiver comprises a transversal desmearing filter and a demodulator coupled thereto. The transversal filters each comprises a plurality of series-arranged delay elements, the time delay .tau. of the element being equal to the sampling period of an input signal. A signal processing arrangement is coupled to taps provided between every two consecutive elements, and also to an input of the first element and to an output of the last element for multiplying in at least each symbol interval T the signals present on the taps by a real individual coefficient determined for each tap and for summing the product signals thus obtained.
2. Description of the Prior Art
Such a transmission system is disclosed in U.S. Pat. No. 3,032,725.
For data transmission use is often made of the public telephone network. Such a network introduces a number of imperfections such as amplitude and phase distortion, frequency offset, phase jitter and Gausian as well as impulsive noise.
The effects of most of these imperfections are reduced or eliminated with the aid of rather sophisticated digital modems. However until now little attention has been paid in the design of these modems to reducing errors caused by impulsive noise. The effects of impulsive noise on the transmission become very pronounced on switched connections and become the more noticeable as the transmission rates increase.
A solution to combat the effects of impulsive noise is obtained by using a smearing filter at the transmitter and a desmearing filter at the receiver of the transmission system. Generally, these filters have a flat amplitude characteristic with a group delay time which increases or decreases linearly with frequency, the sum of the group delay times of both filters being to the greatest possible extent constant. The group delay time of one filter is complementary to the group delay time of the other filter. When ideal filters are used a data signal passing through both filters is only delayed. However a noise pulse only passes through the desmearing filter, so that the energy of such a noise pulse is smeared in time so that its effect on the data signal is considerably reduced at any instant.
An analogue implementation of such filters is described in the article "On the potential advantage of a smearing-desmearing filter technique in overcoming impulse-noise problems in data systems", by R. A. Wainright, published in IRE Transaction on Communication Systems, December 1961. In view of the stringent requirement that the two filters must be perfectly complementary to each other, such an implementation is not so suitable, more specifically because the filter characteristics degrade. For that reason it is already known from U.S. Pat. No. 4,285,045 to use as an alternative digital implementation of these filters, more specifically of the transversal form, for baseband signals.
The smearing efficiency of the prior art filters is however not optimal.